Environmental control systems for jet aircraft function to maintain cabin temperature and pressure at the required comfort level for passengers. Systems operated with auxiliary power and without auxiliary power have heretofore been proposed. Aircraft without auxiliary power operate the main engines at sufficient power to provide compressed air to power the environmental control system when the aircraft is on the ground as well as in the air. However, operating the main engines on the ground results in higher than necessary engine operating costs, including inefficient fuel consumption and increased maintenance and overhaul per aircraft per flight hour. To add auxiliary power also has its price in terms of procurement and installation cost, operating cost and loss of payload due to added weight of the auxiliary power system.
In prior systems having auxiliary power, a prime mover, usually a gas turbine, may mechanically drive a compressor or the compressor may be driven by bleed air from a compressor that is part of the prime mover and is input to the cooling turbine of the environmental control unit. It has been conventional to drive the compressor of the environmental control unit at maximum output to provide a maximum flow rate for the limiting heating or cooling condition and adjust the output flow temperature by flow mixing through a temperature control valve.